By Dr. Evelyn Ng, manager of materials and innovation materials at Callidus, a Score Company and Martin Burgess, engineering lead, Europe, Score
Pressure Oxidation (POX) is a critical process in gold mining, enabling the recovery of gold from refractory ores. It is also one of the most demanding operating environments in the industry. POX circuits combine high temperatures, high pressures, acidic conditions, erosive flow, and oxygen-enriched atmospheres—a combination that places extreme demands on materials and equipment design.
In oxygen-rich environments, materials that are stable in air can ignite or burn rapidly. This introduces fire and explosion risks that require careful control through material selection and component geometry. At the same time, POX equipment must withstand severe corrosion and erosion over extended operating campaigns.
Balancing oxygen compatibility, mechanical strength, erosion resistance and corrosion resistance has historically limited equipment life and driven frequent maintenance intervention. But this is about to change now through a new approach offered by the Australian firm Callidus Group, which Score recently acquired.
Managing corrosion, erosion, and oxygen service risk
Titanium is widely used in hydrometallurgical applications because of its excellent corrosion resistance and mechanical properties. However, in oxygen-enriched POX service, titanium presents a significant fire risk as it ignites easily and burns intensely.
As a result, POX equipment often relies on alternative materials such as carbon steel, copper alloys, nickel-copper alloys, or duplex stainless steels. While these materials are more resistant to ignition, they often lack the corrosion resistance, mechanical strength or durability needed for long-term POX operation.
Industry guidance such as EIGA Doc 13/12 provides recommendations for oxygen service material selection, but these guidelines apply only up to 200 °C. Many POX applications exceed this temperature, leaving operators with limited validated options.
Autoclave vent valves: a high-risk application
One of the most challenging applications in POX plants is the autoclave vent valve. This valve isolates downstream equipment from hot, acidic gases containing oxygen, steam and dilute sulphuric acid, while maintaining tight sealing under extreme pressure and temperature.
In practice, vent valves may experience rapid degradation. Conventional ceramic coatings applied to metal-seated ball valve trim (the internal, wetted, and replaceable parts of a valve that directly contact the process fluid) frequently fail due to coating porosity, delamination, and under-coating corrosion. Once corrosive media penetrates the coating, rapid substrate attack follows, leading to coating delamination and subsequent loss of sealing efficiency and forced valve replacement.
In documented cases, even upgraded alloy selections with traditional ceramic coatings delivered service lives measured in weeks rather than months.
Rethinking protection for POX equipment
To overcome these limitations, Callidus Group, a provider of high-end flow control solutions to the mining industry, developed BM-1600TM, a proprietary process that shifts away from single-layer, mechanically bonded coatings toward engineered, dual-layer protection systems designed specifically for oxygen-rich, acidic environments.
This newer approach combines:
- A fully dense, atomically fused corrosion-resistant layer with a metallurgical bond that prevents delamination, eliminates porosity, and prevents corrosive ingress.
- A low-friction ceramic topcoat to reduce wear and ensure stable sealing under high contact stress.
Unlike conventional thermal spray coatings, the fusion-bonded layer is metallurgically integrated with the substrate, eliminating the interface failures that drive delamination and rapid corrosion.
Oxygen compatibility and safety validation
Because POX involves elevated oxygen concentrations, material durability alone is not sufficient. Components must also show resistance to ignition and combustion under oxygen service conditions.
Materials and coating systems used in this application have been validated using ASTM G124, an internationally recognised test method that determines whether materials can sustain combustion in pressurised oxygen environments.
Testing has demonstrated no sustained burning at temperatures exceeding 230 °C and pressures up to 3,300 kPag in 100% gaseous oxygen, confirming the material’s suitability for severe POX duty beyond the temperature limits addressed by traditional industry guidelines.
Field performance: a step change in service life
Field trials in POX autoclave vent service have shown a substantial improvement in valve trim durability using this dual-layer, fully dense coating approach retrofitted to the client’s existing OEM valve:
- Service life was extended from less than one month to more than one year.
- There was no evidence of coating delamination or spallation.
- Substrate showed no signs of corrosion Sealing integrity was maintained throughout full operating campaigns.
Operational implications for POX plants
Improved trim durability delivers benefits that extend wells beyond component life:
- Improved safety through validated oxygen compatibility
- Reduced unplanned downtime and fewer emergency shutdowns
- Lower maintenance and spare-parts demand
- Greater confidence in isolation performance during maintenance activities
- Improved planning certainty for POX campaigns.
By eliminating porosity and coating-substrate failure mechanisms, this approach allows operators to move beyond the short replacement cycles once considered unavoidable.
A new benchmark for POX equipment protection
The emergence Callidus’ BM-1600TM with its fully dense, metallurgically bonded dual-layer coating systems represents a significant shift in how POX equipment can be protected against corrosion, erosion, and oxygen-related risk.
Rather than managing frequent failures, operators can now specify solutions that deliver predictable, long-term performance in one of the mining industry’s most demanding environments.
This shows that short equipment life in POX service is not an inherent limitation of the process, but a solvable materials challenge.
Biographies of the authors
Dr. Evelyn Ng (PhD Engineeing) is manager of materials and innovation materials at Callidus, a Score Company. Dr. Ng’s pioneering work resulted in the groundbreaking innovations FM-1500 and BM-1600.
Martin Burgess is the engineering team lead for Europe at Score. Fellow of the IMechE, Burgess is an expert in advanced valve engineering.
Download this report to further explore how Callidus’ BM-1600 outperforms traditional material options in severe service.
